The present invention relates to an eyeglass lens processing apparatus that processes a periphery of an eyeglass lens.
In an apparatus that processes the periphery of an eyeglass lens, the eyeglass lens is held by a pair of lens chuck shafts, the lens is rotated by the rotation of the lens chuck shafts, and a roughing tool such as a roughing grindstone is pressed on the lens, whereby the periphery of the lens is roughed. A cup which is a processing jig is fixed onto the surface of the eyeglass lens, and the lens is held by the pair of lens chuck shafts via the cup.
In recent years, a water repellent lens obtained by coating the lens surface with a water repellent material to which water, oil, or the like is not easily attached has become widely used. The surface of the water repellent lens is slippery. Therefore, if the same processing control as in the related art, which is applied to a lens that is not coated with the water repellent material, is applied to the water repellent lens, there is a problem in that so-called “axial deviation” easily occurs in which the rotation angle of the lens deviates with respect to the rotation angle of the lens chuck shaft since the fixed cup slips.
As a method of reducing the “axial deviation”, a technique of detecting load torque applied to the lens chuck shaft and reducing the rotation speed of the lens so as to make the load torque fall within a predetermined value has been proposed (see US2004-192170A1). In addition, a technique has been proposed of rotating the lens at a certain speed and changing the axis-to-axis distance between the lens chuck shafts and the rotation shaft of a processing tool so as to make the cutting amount of the roughing grindstone become approximately constant for one revolution of the lens (see JP2006-334701A). Moreover, as an improved technique disclosed in JP2006-334701A, a technique has been proposed of setting a processing volume per unit time to prevent the occurrence of the “axial deviation” and controlling the axis-to-axis distance by determining the cutting amount per rotation angle of the lens so as to make the processing volume per unit time become constant (see US2010-197198A1).
The control of the rotation direction of the lens in roughing includes a down-cut method in which the rotation direction of the roughing grindstone is opposite to that of the lens, and an up-cut method in which the rotation direction of the roughing grindstone is the same as that of the lens. In the up-cut method, a force pulling the lens to the roughing grindstone side increases, so the “axial deviation” occurs frequently. In the down-cut method, the force pulling the lens to the roughing grindstone is weaker compared to the up-cut method. Accordingly, when the material of the lens is a normal plastic, the down-cut method is used. When the material of the lens is a thermoplastic material (which is a polycarbonate representatively, and Trivex, acryl, and the like are also included in the material), grinding water is not used in roughing (see U.S. Pat. No. 7,617,579B1). As a result, if the down-cut method is used, processing waste discharged in the rotation direction of the roughing grindstone tends to become sticky due to influence from the heat, and the processing waste melted by the heat attaches to the periphery of the lens that has undergone the roughing, which influences processing accuracy of subsequent finishing. In the up-cut method, the processing waste discharged in the rotation direction of the roughing grindstone is discharged to the side of a portion that has not been processed in the roughing. Therefore, it is difficult for the molten processing waste to attach to the periphery of the lens. For this reason, in the case of a lens of a thermoplastic material, the up-cut method is used.